This application proposes to employ an in vitro model of the human placenta, consisting of human placenta-derived cells, cytotrophoblasts, grown on permeable filters. When applied to these filters at appropriate densities, cytotrophoblasts fuse to form syncytiotrophoblasts, mirroring the structure of placental villi. The applicant will characterize the interaction of two drugs with this in vitro model; AZT and interferon, which are known to be effective in inhibiting HIV replication in vitro and in vivo. The studies proposed in this application will characterize: 1) The mechanisms and kinetics of transport; 2) The effects of the drugs on exposed embryos; 3) The cellular integrity of the model placental tissue after exposure to the drugs using ultrastructural analysis; and 4) The physiological response of the placental model tissue to drug exposure using biochemical analyses including an extensive analysis of the interferon system. The primary choice for the in vitro model is a primary culture of cytotrophoblasts from human placenta. Alternatively, more established cell lines may be used (i.e. temperature sensitive SV-40-transformed human placental cells and choriocarcinoma cells) if necessary for some applications. The applicant argues that the proposed work is significant because it will provide a thorough characterization of transepithelial transport of anti-HIV therapies across a placental model using two drugs, AZT and interferon and that the proposed work establishes criteria for transport of other anti-HIV therapies which may be applied in the future.